1. Field of the Invention
This invention relates to an ion implantation apparatus configured to irradiate an ion beam in a ribbon-shape having a larger dimension in an X direction than a dimension in a Y direction substantially orthogonal to the X direction which has scanned in the X direction, or has not scanned in the X direction onto a target, for performing ion implantation. More particularly, the invention relates to an improvement of a means for narrowing the ion beam in the Y direction.
2. Description of the Related Art
FIG. 16 shows a related art of this type of an ion implantation apparatus. The same ion implantation apparatus is described in JP-A-08-115701 (FIG. 1). In the specification and the drawings of the present application, a description is given by taking the case where ions forming an ion beam 4 are positive ions.
In the ion implantation apparatus, an ion beam 4 having a small cross section (e.g., a circular or rectangular spot shape) which will be formed in a ribbon-shaped ion beam is generated from an ion source 2, and the ion beam 4 having the small cross section is mass-separated through a mass separator 6. The mass-separated ion beams are accelerated or decelerated through an acceleration/deceleration device 8, energy-separated through an energy separator 10, scanned in the X direction (e.g., in the horizontal direction) through a scanner 12, and converted into parallel beams through a collimator 14. Then, the ion beams are irradiated onto a target 24 (e.g., a semiconductor substrate) held in a holder 26 to perform ion implantation into the target 24. A path for the ion beam 4 between the ion source 2 and the target 24 is held in a vacuum atmosphere.
The target 24 is mechanically scanned (reciprocatedly driven) along the Y direction (e.g., along the vertical direction) together with the holder 26 within the irradiation region of the ion beam 4 from a collimator 14 by a target driving device 28.
In the specification and the drawings of the present application, a description is given that the traveling direction of the ion beam is referred to as a Z direction. In addition, two directions substantially orthogonal to each other in a plane substantially orthogonal to the Z direction are referred to as the X direction and the Y direction.
In cooperation with the scanner 12 for scanning the ion beam 4 by a magnetic field or an electric field (in this example, a magnetic field), the collimator 14 bends the ion beam 4 scanned in the X direction so as to make it substantially parallel with a reference axis 16 by a magnetic field or an electric field (in this example, a magnetic field), and thus converts the ion beam 4 into a parallel beam. As a result, the ion beam 4 in a ribbon-shape having a larger dimension in the X direction than the dimension in the Y direction (see, FIG. 17, too) is led out. Though it is called as the “ribbon-shape”, it is not meant that the dimension in the Y direction is as thin as paper or cloth. For example, the ion beam 4 has a dimension in the X direction of about 35 cm to 50 cm, and a dimension in the Y direction of about 5 cm to 10 cm. The collimator 14 is referred to as a beam parallelizing magnet when a magnetic field is used as in this example.
The ion implantation apparatus is an example of the case where the ion beam 4 in the ribbon-shape which has scanned in the X direction is irradiated onto the target 24. However, the ion beam 4 in the ribbon-shape may be generated from the ion source 2, and the ion beam 4 in the ribbon-shape may be irradiated onto the target 24 without having been scanned in the X direction.
The transport path for the ion beam 4 is in a vacuum chamber not shown, and held in a vacuum atmosphere. However, gases such as residual gases or out gases are necessarily present though in small amounts. When the ion beam 4 collides against the gas molecules, neutral particles occur. Then, the neutral particles are incident to the target 24, so that a uniformity of an implantation amount distribution is degraded. As a result, an error in implantation amount is caused, or other detrimental effects are caused.
Therefore, the ion beam 4 which is in an energy state to be irradiated onto the target 24 is deflected by an action of a magnetic field or an electric field by means of an ion beam deflector provided near the target 24. Thus, the deflected ion beam 4 and the neutral particles 18 going straight without deflection are separated from each other. As a result, the neutral particles 18 are prevented from being incident to the target 24. The collimator 14 also serves as the ion beam deflector.
The ion beam 4 diverges due to a space charge effect during a travel. From viewpoints of enhancing a throughput of an apparatus, reducing an ion implantation depth in order to miniaturize a semiconductor device formed on the target 24, and the like, the ion beam 4 to be irradiated onto the target 24 is required to have a low energy and a large electric current. However, a divergence of the ion beam 4 due to the space charge effect increases with a reduction in energy and an increase in electric current of the ion beam 4.
The divergence of the ion beam 4 occurs in both the X and Y directions. However, originally, the dimension in the X direction of the ion beam 4 is significantly larger than in the Y direction as described above. Therefore, the detrimental effect by the divergence in the Y direction is larger.
When the ion beam 4 diverges in the Y direction, a part of the ion beam 4 in the Y direction is cut by the vacuum chamber surrounding a path for the ion beam 4 and a mask or the like for shaping the ion beam 4. As a result, a transport efficiency of the ion beam 4 to the target 24 is reduced.
For example, a mask 20 having an opening 22 for passing the ion beam 4 and shaping the ion beam 4 may be disposed between the collimator 14 and the target 24, as shown in FIGS. 16 and 17, or as also disclosed in JP-B2-3567749. The mask 20 may cut an unnecessary bottom portion in the Y direction of the ion beam 4, thereby to shorten the distance L2 missing the target 24 from the ion beam 4.
When the ion beam 4 diverges in the Y direction by the space charge effect, a rate of cutting to the ion beam 4 is increases by the mask 20. Accordingly, an amount of the ion beam 4 capable of passing through the mask 20 is reduced, resulting in a reduction of the transport efficiency of the ion beam 4.
The problem is also present similarly in the case where a ribbon-shaped ion beam 4 is generated from the ion source 2, and the ribbon-shaped ion beam 4 is irradiated onto the target 24 without having been scanned in the X direction.
As a means for compensating for the divergence in the Y direction due to the space charge effect of the ion beam 4, the following means may be considered. An electrostatic lens is provided in a vicinity on a downstream side or an upstream side of the collimator 14 in the path for the ion beam 4.
As shown in FIG. 18, the electrostatic lens 30 includes an inlet electrode 32, an intermediate electrode 34, and an outlet electrode 36 spaced apart from one another in the traveling direction Z of the ion beam 4. The inlet electrode 32 and the outlet electrode 36 are held at a mutually equal electric potential (in FIG. 18, ground potential). The intermediate electrode 34 is applied with a positive or negative direct current voltage V1 from a direct current power source 38. Thus, it is held at a different electric potential from that of the inlet electrode 32 and the outlet electrode 36. The respective electrodes 32, 34, and 36 each individually has a shape corresponding to the shape of the ion beam 4 like a tube or a parallel plate.
The electrostatic lens 30 acts as an einzel lens (which is also referred to as a unipotential lens) It has a function of narrowing the ion beam 4 in the Y direction without changing energy of the ion beam 4 even when the intermediate electrode 34 is applied with either direct current voltage V1 of positive or negative polarity. Incidentally, FIG. 18 shows the state in which the ion beam 4 has not been narrowed for simplification of showing. However, the ion beam 4 is narrowed in actuality.
With the foregoing technique of narrowing the ion beam 4 by the use of the electrostatic lens 30, it is possible to compensate for the divergence in the Y direction due to the space charge effect of the ion beam 4, and to enhance the transport efficiency of the ion beam 4. However, unfavorably, energy contamination occurs like mixing of undesirable energy particles.
When the intermediate electrode 34 of the electrostatic lens 30 is applied with a negative direct current voltage V1, the ion beam 4 is once accelerated in a region between the inlet electrode 32 and the intermediate electrode 34, and then, decelerated in a region between the intermediate electrode 34 and the outlet electrode 36 to return to the original energy. In this acceleration region, when the ion beam 4 collides with residual gases, and neutral particles are generated due to charge conversion, neutral particles having a higher energy than the energy of the incident ion beam 4 are generated. These neutral particles proceed toward the downstream side, which causes the energy contamination of a high energy component.
When the intermediate electrode 34 is applied with a positive direct current voltage V1 as shown in FIG. 18, the ion beam 4 is once decelerated in a region between the inlet electrode 32 and the intermediate electrode 34, and then, accelerated in a region between the intermediate electrode 34 and the outlet electrode 36 to return to the original energy. In this deceleration region, when the ion beam 4 collides with residual gases, and neutral particles are generated due to charge conversion, neutral particles having a lower energy than the energy of the incident ion beam 4 are generated. These neutral particles proceed toward the downstream side, which causes the energy contamination of a low energy component.
Consequently, energy contamination occurs even when the intermediate electrode 34 is applied with either direct current voltage V1 of positive or negative polarity.
Whereas, when the intermediate electrode 34 is applied with a positive direct current voltage V1, as shown in FIG. 18, electrons 39 in an electric field-free drift space where an electric field doesn't exist space on the upstream side and on the downstream side of the vicinity of the intermediate electrode 34) are attracted to the intermediate electrode 34, and vanish. Therefore, when the amount of electrons in the drift space decreases, the divergence due to the space charge effect of the ion beam 4 is intensified. As a result, the transport efficiency of the ion beam 4 decreases.